Field of the Invention
The present invention relates to a power supply circuit that includes a plurality of power supplies having different power supply capabilities.
Description of the Related Art
Recently, there is a demand for an image forming apparatus to consume less energy, so that reduced power consumption is now very important during a power saving mode, and not only during a normal power mode. Japanese Patent Application Laid-Open No. 2010-145653 discusses a power supply circuit that has a configuration like that illustrated in FIG. 5, for example.
In FIG. 5, the power supply circuit includes a power-saving power supply 310 for a power saving mode and a main power supply 320 for a normal power mode, for example. The power-saving power supply 310 for a power saving mode supplies power highly efficiently when the output is a few watts. The main power supply 320 for a normal power mode supplies power highly efficiently when the output is a few hundred watts. A system controller 101 selectively switches these plurality of power supplies based on a state of the image forming apparatus.
Specifically, in the power saving mode, power is supplied from the power-saving power supply 310 to the system controller 101, and in the normal power mode, power is supplied from the main power supply 320 to the system controller 101.
Operation of the circuit illustrated in FIG. 5 will now be described. During the power saving mode, the system controller 101 sets a control signal S1 to a high (Hi) state, and a first switch element 312 is consequently turned on. At this stage, the system controller 101 shuts a relay 301 with a control signal S3, and the main power supply is in an OFF state. The system controller 101 sets a control signal S2 to a low (Low) state and turns off a second switch element 322 as well. In the power saving mode, an output Vcc1 from the power-saving power supply 310 is supplied to the system controller 101 while the other power supply is in an OFF state, so that power consumption is reduced.
Further, when shifting from the power saving mode to the normal power mode, the system controller 101 turns on the relay 301 with the control signal S3 to activate the main power supply 320, and then sets the control signal S2 to Hi to turn on the second switch element 322. Consequently, the output from the main power supply 320 is supplied to the system controller 101. Then, the system controller 101 sets the control signal S1 to Low to turn off the first switch element 312, so that the supply of power from the power-saving power supply to the system controller 101 is stopped. Consequently, in the normal power mode an output Vcc2 from the main power supply is supplied to the system controller 101.
In a power supply configuration like that in FIG. 5, in which power is supplied by connecting the outputs from a plurality of power supplies, when one power supply among that plurality of power supplies is in an ON state and the other power supply is in an OFF state, current flows from the power supply in an ON state to the power supply in an OFF state. Therefore, the flow of current to the power supply in an OFF state is prevented by, as illustrated in FIG. 5, configuring so that diodes 317 and 327 are each connected in series with the outputs from the power supplies, and the cathodes of those diodes are connected to each other.
However, in a conventional circuit configuration in which power (power supply output) is supplied to a load via a diode, a loss of power due to a voltage drop at both terminals of the diode cannot be avoided.
Specifically, after the image forming apparatus has shifted from the power saving mode to the normal power mode, if the amount of power consumed by the system controller 101 increases, the voltage drop in the diode 327 increases. Consequently, the power supply voltage input to the system controller 101 can fall below the minimum voltage required to guarantee operation of the system controller 101. If the output voltage from the main power supply 320 is pre-set at a high level, the voltage drop at the diode 327 also decreases when the amount of current consumed by the system controller 101 is low. Consequently, the power supply voltage input to the system controller 101 can exceed the maximum voltage required to guarantee operation of the system controller 101.
Thus, the greater the fluctuation in the amount of current consumed by the system controller 101, the more difficult it is to satisfy the standards for both the minimum voltage and the maximum voltage required to guarantee operation of the system controller 101.